[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Sat Mar 17 00:45:55 UTC 2012


This is a PS to my preceding reply to this.  If this makes no sense, 
read my other reply first.

Once you get your graphs working, you can select different IP's on the 
peerstats screen as follows.  This may change in future updates to the 

Right click on the graph.

An IP selection screen pops up.

To select a few IP's to show:

      a) click the check boxes for just the IP's you want
      b) click OK

To select all but a few IP's to show:

      a) click the all button
      b) click the check boxes to unselect the IP's you don't want
      c) click OK

To get back to the full list of IP's

      a) right click to bring up the IP selection screen
      b) click none   (Think of it as filtering none of them.)
      c) click OK

There was something else I wanted to mention, and I'm sure I'll remember 
it shortly after sending this.  Anyway, good luck.



On 3/16/2012 7:07 PM, Charles Elliott wrote:
> On the subject of accuracy, has anyone ever really looked at NTPD's offset
> filtering mechanism?  What it does now is sort the last (about 50) offsets
> from smallest to largest and then prunes the smallest or largest, depending
> on which is further away from the average, until there are only N (I forget
> what N is) offset observations left.
> There may be at least two problems with this filtering mechanism.  First,
> there is no apparent theory behind it; I have never seen such a crude filter
> that does not take into account any information inherent in the data.  On
> the other hand, what I don't know about filters would fill all 24 volumes of
> an encyclopedia.
> Second, we know that each offset observation should have arrived about one
> second after the previous one, yet NTPD does not take advantage of that
> knowledge.  There are filters, such as the Kalman filter that uses a
> Bayesian estimation approach to predict the next observation and adjusts it
> according to the prediction when it arrives, that do take advantage of
> previous observations.  Demonstrations of the Kalman filter on the Internet
> show almost spectacular results.  I used a Kalman filter in my clock
> simulation program and the results seemed pretty good.  However, there are
> numerical analysis considerations to programming a Kalman filter as the sums
> and products of observations can become large in a program that runs
> infinitely long.  Also, choosing the parameters of a Kalman filter is
> apparently a black art.
> Would it be worth it to recruit an electrical or systems engineer who
> claimed to know something about filtering data to take a serious look at
> NTPD's data filtering approach?  There has to be some reason that there is a
> significant negative correlation between delay and offset in NTPD.  There
> also has to be a reason that my GPS clock (BU-353, which, when it is working
> well, only has offset ±6 ms from zero) has a difference between about 0 and
> 47 ms from an NTP server on another computer that gets its time from 8 NTP
> stratum 2 servers over the Internet and has remarkably consistent offsets
> ±3.5 ms from zero.  The difference between the GPS clock and the average of
> the stratum 2 servers appears to be a function of the time of day; it is
> large during the mid-part of the day, when the Internet is busy and the
> delay is large and quite variable between servers, and small late in the day
> (right now it is -0.626; 6:55 PM EST), when the delay is smaller and pretty
> uniform for all stratum 2 servers.
> Charles Elliott
>> -----Original Message-----
>> From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
>> [mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On
>> Behalf Of Chris Albertson
>> Sent: Thursday, March 15, 2012 5:22 PM
>> To: unruh
>> Cc: questions at lists.ntp.org
>> Subject: Re: [ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock
>> error.
>> On Thu, Mar 15, 2012 at 2:09 PM, unruh<unruh at invalid.ca>  wrote:
>>> Unfortunately it is not that simple. That rate changes by significan
>>> amounts. Thus the rate you get after a week may be very different
>> than
>>> the rate you get after an hour. That, I submit, is the chief obstacle
>>> to having an accurate clock. And that change in rate does not fit
>> with
>>> the "Allan variance" assumptions (the noise source is not of the type
>>> assumed)
>> You are right about that.  I was going to add in a bit about how to
>> pick the best time to look at the clock tower.  But left it out because
>> the point I was making was only that these things are not NTP
>> specific.   Details after that did not contribute the the main point.
>> Chris Albertson
>> Redondo Beach, California


(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such.  I don't always see new messages very quickly.  If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)

Ron Frazier
timekeepingdude AT c3energy.com

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